In the field of medical image technology such as Positron Emission Tomography (PET) or gamma cameras, as shown in FIG. 1, a plurality of scintillators 130 and associated photomultiplier tubes (PMTs) 110n or avalanche photodiodes (APDs) are usually arranged in a circle of a detector ring 130. Such a detector ring 130 surrounds the patient to be scanned. To conduct a so-called PET scan, a short-lived radioactive tracer isotope, which decays by emitting a positron, is injected usually into the blood circulation of a living subject. After the metabolically active molecule becomes concentrated in tissues of interest, the research subject or patient is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.
As the radioisotope undergoes positron emission decay, it emits a positron, the antimatter counterpart of an electron. After traveling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of gamma photons moving in almost opposite directions. These are detected when they reach one of a plurality of scintillation crystals in the scanning device, creating a burst of light detected by an array of photo detectors comprised either of photomultiplier tubes (PMTs) or silicon avalanche photodiodes (Si APDs). Generally a plurality of PMTs or APDs are arranged in a matrix and assigned to a single scintillator detector as shown in the enlarged section 160 in FIG. 1. The scintillator detector can be a single scintillation crystal or can be, as shown, a matrix of scintillator crystals 130 coupled to the PMTs via a light guide 135. To be able to increase the resolution of the system without the high costs of 1:1 coupling, the number of photo detectors per block is generally significantly lower than the number of scintillation crystals. For example, a block detector may have a plurality of photo detectors with, for example, 4 or 9 PMTs or APDs arranged in a 2×2 or 3×3 matrix behind an array of scintillation crystals. Other arrangements with more or less photo detectors are possible. Thus, the event localization may be determined or interpolated by such a scintillation block detector by processing the associated photo detector signals. This can be done by analog filtering, integration, and multiplication of weighted combinations of the photo detector signals or by using digital algorithms that use discrete time sample points of signals obtained directly from the photo detectors. The PET technique depends on simultaneous or coincident time detection of the pair of photons.
On the right side of FIG. 1a block diagram is shown of the typical architecture of detectors and associated analog-to-digital-converters in a conventional system. Each matrix of PMTs or APDs produces a plurality of signals that need to be processed to generate an image from a plurality of single events that are detected by a PMT. To determine the position of a detected annihilation, the system needs to accurately measure the timing and energy of both detected photons. Consequently a high amount of data has to be produced by the respective measurement circuits.
For example, as shown on the right side of FIG. 1, each scintillator has an associated matrix of detectors, such as PMTs 1101 . . . 110n. Each signal of each PMT 1101 . . . 110n is first amplified, for example by an associated discriminator/filter 1201 . . . 120n which are capable of optimizing desired photoelectron signal response while minimizing noise and amplifying the respective signals for further processing. The output signal of all discriminators/filters 1201 . . . 120n are then concurrently converted into discrete-time digital signals by associated analog-to-digital converters (ADC) 1401 . . . 140n. The sample rate frequency for each ADC is provided at terminal 150. Using this digital processing architecture requires n independent ADC channels with peripheral circuitry to concurrently sample each of n photo detector signals per block. This causes high costs on the design of a detector unit.